1. Field of the Invention
The invention relates generally to vacuum processing chambers and more particularly to parallel plate plasma processing chambers.
2. Description of the Prior Art
An electrically neutral quantity of highly ionized gas composed of ions, electrons and neutral particles is known as a plasma. Since a plasma contains ions, some of its particles are in an elevated energy state and the excess energy which they possess may be transferred either or both to other matter in the plasma or to solid objects contacted thereby. Depending upon the atomic and/or molecular composition of the plasma, its energy, its density, the composition of the material which it contacts and other factors, this energy transfer may occur either through mechanical collision of energetic particles, by chemical reaction between or among particles or by a combination of mechanical collision and chemical reaction. Depending upon the precise nature of the plasma and the configuration of the chamber in which it is formed, these processes may either deposit or remove material from the surface of solid objects contacted by the plasma. For this reason it has been recognized for some time that plasmas may be exploited in manufacturing processes to mechanically and/or chemically alter the surface of a workpiece.
If a plasma interacts with a solid object solely through mechanical collisions, such as occurs in a chemically inert plasma as might be established in argon gas, the process is generally referred to as sputtering. In sputtering, atoms are removed from a solid surface by accelerated gas ions much like in a billiard ball fashion by striking the surface and ejecting material therefrom. After a collision, both the incident particle and the ejected material respond independently to the forces to which they are respectively subjected. For this reason there is no need to replenish or renew the gas in which the sputtering plasma is established except as necessary to maintain the purity of the inert gas in the plasma and to sweep away gaseous ejecta.
If, alternatively, the plasma is established in a gas containing chemically reactive species and the plasma interaction occurs primarily through a chemical reaction between particles, the process is generally referred to as either plasma etching or plasma deposition depending upon the net effect produced on a workpiece's surface. Generally, a plasma etching process is designed so that the products of the reaction between the gas of the plasma and the surface of the workpiece remain in the gaseous state. Conversely, a plasma deposition process is designed so that the product of the chemical reaction condenses to the solid state on the surface of the workpiece. Thus, because the chemical composition of the plasma changes as the reaction proceeds, the gas being energized into the plasma state usually flows continuously through the reaction region of the chamber in which the plasma is formed. This flow of gas serves both to constantly replenish the supply of unreacted atoms and/or molecules and to carry off the waste products of the reaction. While a plasma process may be designed to optimize a chemical reaction occurring therein, it is virtually impossible to totally eliminate the mechanical process of sputtering if the plasma contacts the surface of solid material.
In the semiconductor fabrication industry, etching is routinely employed as a fabrication process to establish patterns which delineate the various surface regions of a semiconductor component or integrated circuit. The lines and spaces of the pattern to be established in the surface of a semiconductor workpiece during its fabrication are generally first defined by coating that surface with a film of radiation sensitive material generally referred to as photoresist. This film is then exposed to radiation such as light, x-rays or electron-beams having the desired pattern to form a latent image therein. The latent image thus established in the film is then developed in much the same fashion as lithographic plates are processed in the printing industry. Development of the latent image results in the formation of open areas in the film. When this patterned surface of the semiconductor workpiece is subsequently exposed to an environment which preferentially removes material underlying the open areas, the covered regions remaining unattacked become raised above the surrounding etched regions. Historically, the semiconductor industry has employed wet chemical etching to perform this process.
Recently, plasma etching has been increasingly adopted as a manufacturing process in the semiconductor industry because of its ability to produce fine line geometries needed to delineate the components of large scale integrated circuits. The geometric advantage of plasma etching over wet chemical etching occurs because anisotropic plasma etching reactions can be obtained, i.e. plasma processes may be designed in which the etch rate perpendicular to the workpiece surface exceeds the etch rate parallel thereto. This relatively lower lateral etch rate reduces the photoresist undercutting typical of isotropic wet etching processes. In addition, because plasma etching is based primarily upon the chemistry of the gases in which the plasma is formed and their reaction with the surface of the workpiece, it is possible to alter plasma parameters to produce a continuous transition between anisotropic and isotropic etching. Furthermore, by controlling the ion energy and density of the plasma, most plasma etching systems can employ both plasma etching and sputtering to remove material from the surface of a workpiece. Such processes combining both chemical and physical processes for material removal, generally referred to as reactive ion etching, allow significantly greater control over the widths of lines and profiles of their edges than obtainable from wet etching. Thus, by proper control of plasma processing parameters it is possible to sequentially pass from an anisotropic to an isotropic etch or conversely and thereby to control the slope of an etched wall.
Two types of plasma processing chambers, barrel or parallel plate, are generally used for semiconductor etching. In barrel type plasma processing chambers, disk-shaped semiconductor wafers are positioned so that their planar surfaces are aligned perpendicular to and displaced along the central cylindrical axis of a barrel-shaped reaction chamber. A radio frequency electrical current is then coupled into the gas within the processing chamber by means of a plurality of electrodes located around and just within the cylindrical outer surface of the chamber. The chemically active species generated within plasma established by this electrical current, initially located around the periphery of the processing chamber, then diffuse through a perforated etch tunnel wall surrounding the parallel wafers to reach reaction sites on their surfaces. However, since barrel type plasma etchers generally produce isotropic etching reactions they are unsuited to high density, small feature integrated circuit fabrication. Alternatively, parallel plate etching chambers in which the planar surface of a wafer is positioned parallel to and between parallel planar electrodes may be operated so as to produce anisotropic etching. Anisotropic etching may be obtained from parallel plate chambers because the electrical field associated with the radio frequency current flowing between the parallel electrodes imparts a directional aspect to the reactants created within the plasma.
Generally, parallel plate plasma processing chambers have been constructed by securing opposing electrically insulated parallel plates within the walls of a much larger vacuum chamber. These plates are then electrically connected to a radio frequency current generator located outside the chamber. This chamber is further provided with a means for controlled admission of the reactant gas with the waste products of the reaction generally being removed by a vacuum pump. After assembly, such a chamber must first be mechanically adjusted for proper operation. In particular, the parallelism of the plates must be adjusted to produce uniform etching across the surface of a wafer and the spacing must be adjusted to obtain efficient operation of the plasma reaction. Adjustment of these plates is generally difficult and awkward. In general, adjustment of plate spacing involves evacuating the chamber, establishing a plasma therein, determining that the spacing is incorrect, extinguishing the plasma, breaking vacuum on the chamber and then changing the plate spacing which, of course, generally entails some risk of adversely affecting the adjustment of plate parallelism. After each adjustment has been made, the first portion of this procedure must be repeated to determine whether the adjustment was made correctly. If an unsatisfactory adjustment was made, the remainder of the procedure must be repeated so that a plate spacing may be further altered. The process for adjusting plate parallelism is similar to that for plate spacing.
In addition to being difficult to adjust, plasma processing chambers constructed in the foregoing manner also exhibit other undesirable characteristics. In such a chamber, the region in which the plasma is formed may include not only the region immediately between the parallel plates but also may include the opposite sides, i.e. backsides, of the plates. A plasma having such an extent can exhibit a phenomenon known as "backside sputtering" wherein material is removed from the backside of the parallel plates. Material thus removed from the backsides of the plates may have a deleterious effect on the plasma reaction and/or may be deposited on the surface of the workpiece. Furthermore, existence of a plasma extending beyond the region immediately between the parallel plates, even if it does not result in deleterious backside sputtering, indicates that radio frequency energy being supplied to the processing chamber is being wasted thus reducing the efficiency of the etching process.